There are a variety of applications for inspecting surface quality of manufactured products. The ultimate quality of the surface not only determines marketability of the product, but finish durability as well. A variety of systems have been developed to inspect a multiplicity of surfaces, such as painted panels in the automotive field, appliance, furniture and aircraft fields, the inspection of painted rolled sheet and high speed steel and aluminum sheet. A variety of applications of these products demand superior surface finish. Until the advent of machine vision, products were inspected by human inspectors. Out of necessity, this inspection was carried out at low line speeds to prevent blurring to the eye.
A variety of electronic devices have been developed which are either operator dependent or completely automated to detect surface finish imperfections and either signal or document such imperfections. This permits various types of assembly lines and manufacturing lines to operate at normal processing speeds.
A system which interfaces with the operator is disclosed in U.S. Pat. No. 4,539,561. The system is adapted to detect surface quality of cast aluminum strip. At least one video camera is used in the detection process. The system is adapted to restrict transmission from the camera to the TV monitor, so that only defective sections of the cast aluminum strip are shown on the TV camera. This considerably reduces the time that the operator must concentrate on the TV camera to detect surface imperfections and hence provides a more acute attention span for areas of the cast aluminum strip where surface imperfections are detected by the system. The signal from the video camera is delimited with an upper and lower signal strength Defective surface conditions cause the signal from the video camera to exceed or fall below these limits. At that point, recording and transmission of the image to the TV monitor is triggered. The operator can then view the imperfection and decide on a course of action to deal with the imperfection in the cast strip.
U.S. Pat. No. 4,863,268 discloses a surface detection system which is particularly useful in detecting irregularities or flaws in the surface of automobile panels. The source of light and camera are both oriented in the same direction. A reflective surface is used to direct light reflected from the surface being inspected back to the camera. By rereflecting the light over the surface being inspected, a shadow may be produced as observed by the camera which is indicative of the quality of the surface finish. Although the system is compact, it provides, however, for only a single light source and a single camera which limits the types of surface imperfections which can be detected. As such the system is not particularly useful in detecting surface imperfections on high speed conveyed sheet material.
A similar system is disclosed in U.S. Pat. No. 4,629,319 involving a single TV camera and a single light source. Both the light source and camera may be mounted on robotic arms. The light source may be altered with a grill to provide a plurality of light lines all of the same intensity. By virtue of mounting the lamp and camera on separate robotic arms, their angular position may be varied as the system traverses the surface being inspected to enhance the highlighting of the surface imperfections. The system is particularly useful at low angles to detect surface imperfections, such as poorly oiled panels. However, this greatly expands the size of the system. Therefore it is desired to inspect the panel with an angle of incidence for the light beam of greater than 50.degree. relative to the planar surface of the sheet or panel being inspected. In order to operate at these higher angles, manipulation of the robotic arms and, in turn, varying the position of the lamps and camera are required to detect surface imperfections. Although the system is effective in detecting surface imperfections on the order of mm in section on complex car panel shapes which are generally smooth surfaces, it is a relatively expensive system to install.
U.S. Pat. No. 4,675,830 discloses a video surface inspection system particularly adapted to sense imperfections in high speed aluminum sheet. The system is capable of detecting surface imperfections in sheet moving at speeds up to 3600 feet per minute. The system employs a plurality of lamps which direct light beams transversely of the motion of the sheet. Two light sources positioned on opposite sides of the sheet provide diffuse lighting while two other lamps positioned above and to each side of the sheet provide specular lighting. With the lamps positioned to each side of the sheet and transversely lighting the sheet, a plurality of video cameras are positioned above the sheet to detect highlighted surface irregularities in the moving sheet. The system is devised to provide diffused and/or specular illumination which gives a more homogeneous coverage of the web surface where it is thought that a combination of diffuse and specular illumination can be used to highlight variations in the surface being examined. It is preferred that the lamps emit a longwave invisible (red) portion of the spectrum to match the image sensors of the video cameras and as well to improve the work environment.